I have TLDCN 2008 and here's how mine works - this might be in some of the help or tutorials. In the Trip Planner, click on Customize, then click on Road/Preferences. These are the tables that are used for your trip planning based on the Short, Fast or Customize option you select.
The Short option picks the shortest route in miles to get to a location, regardless of the speed or road type you prefer. You'll find that you may end up on some pretty slow roads to get to your destination but it will be shorter in miles.
The Fast option picks the fastest route (interstates versus streets) to get you to a location based on the speeds you have entered in the Driving Speed table for each road type. This might take you a longer route in miles to get between your waypoints but will generally get you there faster because of the types of roads and the speeds listed for them.
Customization uses the Road Preferences based on the slider bar. For example, if you choose Highways and state roads over Interstates, you may find that your trip might be longer or slower than the other options but at least you are on the road types you prefer.
The Other Preferences on that page will also affect your route for all trip types. So will the "restrictions" from the Options from another tab regarding rig length especially if it takes you through a State that won't fit your rig.
Keep in mind that in all cases there is no accounting for traffic conditions (rush hours) or other unknowns not available to the the "program" like temporary road closures, road construction (flagman controlled) or road conditions (curvy or in bad shape). These options are only an attempt to plan trips on YOUR preferences using general guidelines. You might still have to include or exclude areas to meet your trip needs by adding waypoints where you want to be or creating map exclusions to avoid areas you don't want.
I hope this helps trying to understand how this works and how you can control your trip planning. I still examine the route and must make changes even though I thought I picked the right parameters. But other (more expensive) mapping programs will have their nits, too. So I wouldn't get too frustrated that this is an exact science. It's all about the algorithms, but that's beyond the scope of this topic.